Method for making spherical particles

ABSTRACT

A process and apparatus for spheridizing irregularly shaped minute particles, and the spheres produced thereby, in which a thin carbonaceous coating is applied to the particles in a unique manner, and in a preferred embodiment the particles are then advanced through successive fluidizing beds. The first bed has an inert atmosphere and is maintained at an elevated temperature sufficiently high to allow surface tension to shape the particles into spherical form while in a fluidized condition in the first bed. The spherical particles are then advanced through successive additional beds where they are cooled to an intermediate temperature sufficient to solidify the particles, are subjected to an oxidizing atmosphere which completely removes the coating, and are then further cooled while being maintained in a fluidized condition. The inert gaseous atmosphere within the first bed is continuously withdrawn and recycled through the system.

BACKGROUND OF THE INVENTION

This invention relates to a novel sphere making process and moreparticularly to such a process for making spheres from minute particlesof glass or other sphere forming material.

Glass beads and other spherical particles manufactured in accordancewith the invention have numerous industrial and commercial applications.In many cases the beads are used to provide a reflecting surface, suchas in lane marking for highways, for road and advertising signs, motionpicture screens, etc. Other uses for the beads include applications inwhich their reflecting properties are of little moment, as in cases inwhich the beads are employed as fillers for plastic materials, forimpact treatment and peening of metal surfaces, or for variouselectrical uses. The diameter of the beads may vary widely andillustratively ranges from about 6.0 millimeters down to about 1 micron.

Various processes and apparatus have been employed heretofore tomanufacture glass spheres. As an illustration, it often has been commonpractice to introduce irregularly shaped glass particles into avertically disposed draft tube which is open at its upper end and isprovided with a well-distributed gas flame adjacent its lower end. Theparticles are carried upwardly by the combustion gases into an expansionchamber or stack mounted above the draft tube. During their upwardmovement, the particles become soft and are shaped by surface tensioninto a substantially spherical configuration to form glass beads. For amore detailed discussion of representative bead manufacturing systems ofthis type, reference may be had, for example, to U.S. Pat. No. 2,334,578granted Nov. 16, 1943, to Rudolf H. Potters, U.S. Pat. No. 2,619,776granted Dec. 2, 1952 to Rudolf H. Potters, U.S. Pat. No. 2,945,326granted July 19, 1960 to Thomas K. Wood and to U.S. Pat. Nos. 3,560,185and 3,560,186 granted Feb. 1, 1971 to Arthur G. Nylander.

In other cases glass spheres have been produced directly from a streamof molten glass as shown, for example, in U.S. Pat. No. 3,279,905granted Oct. 18, 1966 to Thomas K. Wood et al. Still further spheremaking processes of the type previously employed include the use of arotary kiln. In these latter processes the crushed glass particlescustomarily are coated with a resin or other binder and a material suchas graphite to provide a protective coating and/or matrix around eachparticle as the spheres are formed. Processes of this latter type aredisclosed in U.S. Pat. No. 3,597,177 issued Aug. 3, 1971 to CharlesDavidoff and U.S. Pat. No. 2,461,011 issued Feb. 8, 1949 to N. W. Tayloret al.

The prior processes and apparatus employed in the manufacture ofspherical particles such as glass beads have exhibited certaindisadvantages. As an illustration, the overall thermal efficiency ofmany such prior systems was comparatively low, with the result that themanufacturing cost of the beads was excessive. In addition, and this hasbeen of special moment in processes and apparatus which used a verticaldraft tube, the thermal efficiency was further impaired because of theneed to use a portion of the available energy for the vertical transportof the particles, and the temperature gradient within the tube resultedin the production of spheres which occasionally exhibited a lack ofroundness and had other defects. It was also necessary to carefullycontrol the population density of the particles in order to minimize theincidence of collisions between particles which detracted from thequality of the product. The equipment previously employed to produceglass spheres was large in size and had additional disadvantages whichfurther detracted from the efficient and economical manufacture of thespheres on a continuous large volume basis.

Other prior processes and apparatus, such as those utilizing rotarykilns and similar equipment, had the disadvantage that the coatingmaterials employed required either a binder for the protective coatingor a matrix of substantial mass that needed to be heated in addition tothe particles. A further disadvantage of processes and apparatus of thislatter type was the fact that the coating material had to be removed ina costly mechanical process like washing, etc. to achieve a coating freeproduct.

SUMMARY

One general object of this invention, therefore, is to provide a noveland economical process for producing glass beads or other sphericalparticles.

More specifically, it is an object of this invention to provide such aprocess in which the available heat is utilized in a more efficient andless expensive manner than has been attainable heretofore.

Another object of this invention is to provide a process for producingspherical particles in which the resulting particles exhibit extremelygood uniformity and roundness characteristics.

A further object of the invention is to provide a particle producingprocess in which an extremely fine and uniform coating is applied to theparticles without the use of binders or matrices.

A still further object of the invention is to provide a particleproducing process of the character indicated wherein the coating isremoved to produce optically clear particles without washing ormechanically removing the coating.

Still another object of the invention is to provide a new and improvedsystem for manufacturing glass beads that is economical and thoroughlyreliable in opera- tion.

In one illustrative embodiment of the invention, a multiplicity ofcrushed glass particles is introduced into a fluidizing bed. An inertgas or other fluidizing material is directed into the bed to suspend theparticles in a fluidized condition, and the particles are heated to anelevated temperature sufficiently high to allow surface tension to shapethe particles into spherical form. The particles are thereafter cooledwhile continuing their fluidization for a period of time sufficient tocause the setting of the particles in the form of spheres.

The use of a fluidizing bed to produce or otherwise treat the particlescomprises a particularly advantageous feature of a number of preferredembodiments of the invention. The bed serves to confine the particleswithin an area that is much smaller than that of most of the spheremaking systems employed commercially heretofore, with the result thatthe amount of heat loss during the spheoridization of the particles issubstantially reduced. In addition, the more even heat distributionwithin the bed enables the production of spheres that have improvedroundness and size characteristics.

In accordance with another feature of the invention, in severaladvantageous embodiments, prior to the time they reach their softeningtemperature the particles are provided with a thin coating of protectivematerial. In cases in which the particles come in contact with oneanother during their formation into spheres, the coating serves toprevent the particles from agglomerating or otherwise sticking together.The coating preferably comprises an oxidizable carbon which adheres tothe particles even in vertical draft tube or rotary kiln type systems.

In accordance with a further feature of several good embodiments of theinvention, after spheoridization the coated particles are exposed to anoxidizing atmosphere. The particles are maintained in the atmosphere fora period of time sufficient to burn off or otherwise oxidize and removethe coating in a manner such that the resulting spheres are opticallyclear and have good retroreflective properties.

The present invention, as well as further objects and features thereof,will be understood more clearly and fully from the following descriptionof a preferred embodiment, when read with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a process and apparatus for producingglass beads in accordance with one illustrative embodiment of theinvention.

FIG. 2 is a longitudinal vertical sectional view taken along the line2--2 in FIG. 3 and illustrating a fluidizing bed and associatedcomponents utilized in the process and apparatus of FIG. 1.

FIG. 3 is a transverse vertical sectional view taken along the line 3--3in FIG. 2.

FIG. 3a is a horizontal sectional view taken along the line 3a--3a inFIG. 2.

FIG. 4 is a vertical sectional view of another fluidizing bed utilizedin the process and apparatus of FIG. 1.

FIG. 5 is a horizontal sectional view taken along the line 5--5 in FIG.4.

FIG. 6 is a longitudinal vertical sectional view of a third fluidizingbed utilized in the process and apparatus of FIG. 1.

FIG. 7 is a transverse vertical sectional view taken along the line 7--7in FIG. 6.

FIG. 8 is a vertical sectional view of a fluidizing bed utilized in aprocess and apparatus for producing glass beads in accordance withanother illustrative embodiment of the invention.

FIG. 9 is a horizontal sectional view taken along the line 9--9 in FIG.8.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, there is shown a process andapparatus for manufacturing glass beads from minute crushed glassparticles. In the illustrated embodiment the crushed particles comprisea conventional soda-lime-silicate glass, but the process and apparatusmay be employed with substantially equal facility to produce spheresfrom other types of glass, from plastics or from substantially any otherparticulate material that has the property of becoming spherical throughsurface tension or other means upon the application of heat. The processand apparatus illustrated in the drawings have particular utility in themass production of glass beads through the use of one or more fluidizingbeds. As will be explained in more detail in the ensuing discussion,however, certain of the features of the invention also are applicable tothe manufacture of the beads by means of vertical draft tubes, rotarykilns or other types of bead-making systems.

In the process and apparatus of FIG. 1 a multiplicity of crushed glassparticles are continuously fed to a tumbler 10 through an infeed conduit11. The tumbler 10 is of conventional construction and also includes aninfeed conduit 12 for receiving a suitable coating material. Thiscoating advantageously comprises an oxidizable adherent carbonaceousmaterial, of a type to be described in more detail hereinafter, and isthoroughly mixed with the glass particles within the tumbler 10 toprovide an extremely thin but complete coating on each particle. Nobinders or matrices are added to the mixture, but the extremely fineparticle size and adherent properties of the coating material contributeto the realization of a smooth and uniform coating around each particle.

The thus coated glass particles are led from the tumbler 10 through aconduit 13 and a valve 14 to a sphere forming enclosure in the form of afluidizing bed 15. As best shown in FIGS. 2 and 3, the conduit 13 entersthe fluidizing bed 15 adjacent the left or infeed end of the bed's upperwall 19. The bed 15 also includes a lower wall 20, longitudinal sidewalls 22 and 23, and transverse side walls 25 and 26, which are eachfabricated from a refractory heat-insulating material. These walls forma longitudinally extending rectangular enclosure for the variousinternal components of the bed.

Mounted within the fluidizing bed 15 are two long and comparativelynarrow channels 30 and 31. The channels 30 and 31 are arranged in sideby side parallel relationship with each other in position to receive thecoated glass particles from the particle conduit 13. The conduit 13 isprovided at its lower end with a Y connection to form branch conduits13a and 13b which communicate with the channels 30 and 31, respectively.The channels 30 and 31 are each provided with a series of baffles 32(FIG. 3a). The baffles 32 are in the form of vertical plates which liein planes transverse to the longitudinal direction of the channels andextend alternately from opposite sides of each channel to provide asinuous path for the particles moving along the channel.

Heating zones 36, 37 and 38 are provided within the fluidizing bed 15.The temperature of these zones is controlled in part by heating elements39, 40 and 41 within the bed 15. As best seen in FIGS. 3 and 3a, theheating elements 39 and 41 are located adjacent the respectivelongitudinal side walls 22 and 23 of the bed 15, while the heatingelement 40 is centrally located midway between the two channels 30 and31. The bed 15 additionally includes an outfeed zone 45 located at thedownstream or discharge end of the bed. The zone 45 is separated fromthe zone 38 by a vertically disposed weir plate 46 which extendstransversely across each of the channels 30 and 31.

An adjustable weir plate 47 is interposed between the outfeed zone 45and the transverse wall 26 at the discharge end of the fluidizing bed15. The weir plate 47 is provided with a central opening defined by ahorizontal ledge 48 and is slidably positioned for vertical movementwithin a discharge conduit 49 which extends through the lower wall 20 ofthe bed 15. This discharge conduit communicates with the channels 30 and31 of the bed 15 by means of a suitable Y connection (not visible in thedrawings). The weir plate 47 may be moved either upwardly or downwardlywith respect to the conduit 49 to vary the position of the ledge 48within the conduit.

A perforated bottom plate 51 is supported within the fluidizing bed 15 ashort distance above the bottom wall 20. Five gas inlet conduits 53, 54,55, 56 and 57 extend through the bottom wall 20 of the bed 15 and arespaced along the longitudinal center of the bed. These inlet conduitsare arranged to admit fluidizing gas into the space between the wall 26and the bottom plate 51 and then through the perforations in the bottomplate to the two interior channels 30 and 31.

Referring again to FIG. 1 of the drawings, the fluidizing gas isintroduced into the system through an inlet conduit 60. The gasadvantageously comprises nitrogen or other gas which is sufficientlyinert that it will not react with either the coating material or theparticles being spheoridized at the temperatures used in the system. Theincoming gas is directed through an inlet valve 62 and a blower 63 to aheat exchanger 65. From the heat exchanger 65 fluidizing gas is admittedto a preheater 67 and then to successive heaters 69 and 70.

A branch conduit 72 is connected to the inlet conduit 60 between theheaters 67 and 69. The conduit 72 leads to two valves 73 and 74 inparallel relationship with each other, and these valves in turn areconnected to the inlet conduits 53 and 54. A second branch conduit 76 isconnected to the conduit 60 between the heaters 69 and 70. The conduit76 leads through a valve 77 to the gas inlet conduit 55. The remaininggas inlet conduits 56 and 57 are connected to the conduit 60 by a branchconduit 78 on the downstream side of the heater 70. The flow offluidizing gas through the heater 70 and the branch conduit 78 iscontrolled by a valve 79.

The arrangement is such that the fluidizing gas within the conduit 60 ispreheated by the heat exchanger 65 and the preheater 67, and a portionof the preheated gas is then introduced into the zone 36 of thefluidizing bed 15 through the branch conduit 72 and the inlet conduits53 and 54. Another portion of the preheated fluidizing gas is furtherheated by the heater 69 and is introduced into the zone 37 of the bed 15through the branch conduit 76 and the inlet conduit 55, while a thirdportion of the preheated fluidizing gas is further heated by the heater70 and is introduced into the zones 38 and 45 of the bed through thebranch conduit 78 and the inlet conduits 56 and 57.

The fluidizing bed 15 is provided with a gas outlet conduit 80 forcontinuously withdrawing inert gas from adjacent the upstream end of theupper wall 19 (FIG. 2). The outlet conduit 80 is connected to a cycloneseparator 84 which serves to separate dust and other particulatematerial from the hot inert gas coming from the fluidizing bed 15. Theparticulate material is returned to the bed 15 through a valve 82 and areturn conduit 85, while the withdrawn gas is led through a conduit 87to the heat exchanger 65 where it is used to partially preheat the freshinert gas within the inlet conduit 60. From the heat exchanger 65 thewithdrawn gas proceeds to a cooling unit 90 which is supplied withcooling water through a conduit 92. The cooled gas then enters a bagfilter 95 having a blowback nitrogen supply conduit 96 and a dust bin 97which collects residual particulate material within the gas. A conduit99 directs the gas from the bag filter 95 to a further filter 100, andthis latter filter is connected by a conduit 102 to the inlet conduit 60between the valve 62 and the blower 63. The thus cooled and filtered gasis admixed with the fresh fluidizing gas in the conduit 60 and isrecycled through the system.

A conduit 103 is connected to the fluidizing gas conduit 60 by a valve104. The conduit 103 joins the conduit 60 between the heat exchanger 65and the preheater 67 and is used to supply fluidizing gas to anintermediate fluidizing bed 105 which receives the coated glass spheresfrom the discharge conduit 49. The fluidizing bed 105 serves as a sealerbed to isolate the inert atmosphere within the bed 15, and it alsoeffects a partial cooling of the spheres. To insure the free flow of theparticles falling through the discharge conduit 49 into the bed 105, thebed is provided with an outlet conduit 106 which leads to a pump 108 andthen to the separator 84. The pump 108 continuously withdraws inert gasfrom the bed 105 and thereby prevents the build-up of excessive pressurewithin the bed.

From the sealer and intermediate cooler bed 105 the coated spheresproceed through a discharge conduit 107 to an oxidizer fluidizing bed110. As best shown in FIGS. 4 and 5, the oxidizer bed 110 includes acasing 112 of refractory heat-insulating material which encloses acylindrical shell 113. The shell 113 is provided with a perforatedbottom plate 114 in spaced relationship with the bottom wall of thecasing 112. An air inlet conduit 115 extends through the bottom wall ofthe casing to admit fluidizing gas into the space between the bottomwall and the plate 114 and then through the perforations in the plate tomaintain the coated spheres within the shell 113 in a fluidizedcondition in an oxidizing atmosphere. The shell 113 provides anenclosure for the coated spheres and includes a vent conduit 116 whichextends upwardly from the top of the shell.

The fluidizing bed 110 is supplied with air or other oxidizing gas froman inlet conduit 120 (FIG. 1). The incoming gas proceeds through an airfilter 121 and a blower 122 to a valve 124 and then through a heater 126to the inlet conduit 115.

Fluidized particles from the fluidizing bed 110 are discharged through aconduit 128 to a cooler fluidizing bed indicated generally at 130. Asbest shown in FIGS. 6 and 7, the cooler bed 130 includes a rectangularmetal casing 132 which is surrounded by a cooling jacket 134. The jacket134 is supplied with water or other cooling fluid through an inletconduit 135, and the cooling fluid is withdrawn through an outletconduit 136. Spaced a short distance above the bottom of the casing 132is a perforated plate 138. The space beneath the plate 138 is suppliedwith air or other fluidizing gas at room temperature from an inletconduit 139 which is connected through a valve 142 (FIG. 1) to thesupply conduit 120 between the blower 122 and the valve 124. Thefluidizing gas is continuously discharged from the cooler bed 130through a vent 144 which communicates with the vent 116 leading from theoxidizer bed 110.

The irregularly shaped particles introduced into the tumbler 10 compriseparticles of glass or other vitreous material. In addition to soda limeglass commonly used for highway striping, glass having a higher index ofrefraction such as the titanium glasses, for example, may be employedwith substantially equal facility. The particles may be screened, ifdesired, to limit the product to a particular size range, or they may betreated in accordance with the process to create glass spheres ofvariable sizes which may then be screened, if desired, to provide beadsof a particular size range. The process also may be used to producespheres from plastic particles or substantially any other materialhaving the capability of becoming spherical upon the application ofheat. One of the advantages of the process is that it has the capabilityof producing larger diameter spheres than many of the processes employedcommercially heretofore. In the prior vertical draft tube systems, forexample, the spheres customarily range in diameter from about 1 micronto a maximum of about 1.0 millimeters, but with the process of thepresent invention good quality spheres are formed which have a diameterrange of anywhere from about 1 micron to about 6.0 millimeters.

The irregularly shaped particles are thoroughly mixed in the tumbler 10with an oxidizable adherent protective coating of extremely fineparticle size. Although a wide variety of coating materials may beemployed which meet these criteria, particularly good results areachieved with coatings of carbon black. Boron nitride, the silanescontaining carbon atoms, and other carbonaceous material that is notwetted by soft or molten glass also may be employed with good effect.Among the carbon blacks useful as coating materials are those availablecommercially and identified as furnace black.

The quantity of coating material employed should be sufficient toprovide a complete and uniform coating around each glass particle. If anexcess of coating material is applied to the particles, however, theexcess material does not improve the efficacy of the coating and ismerely wasted. For crushed glass particles ranging in size from 18 to 40mesh U.S. Standard, and for the carbon blacks used thus far, the amountof coating material per pound of particles preferably should range fromabout 0.5 grams to about 2 grams, and particularly good results areachieved in cases in which the coating is applied in the ratio of about1.0 grams per pound of particles. Below about 0.5 grams per pound thematerial is insufficient to completely coat each particle, while aboveabout 2 grams per pound a satisfactory product is achieved but theexcess coating provides no further beneficial effect. For particlessmaller than 18-40 mesh, a proportionately greater amount of coatingmaterial is employed because of the increased surface area of theparticles. Conversely, particles above this particular range requirecorrespondingly less coating material. The quantity of coating materialused for a particular run is inversely proportional to the surface areaof the particles in a substantially straight line relationship. To meetthese criteria the amount of coating material on the glass particlesadvantageously ranges from about 0.1% to about 0.5% by weight.

The use of an adherent coating material of this character enables therealization of a smooth and uniform coating on each particle without thenecessity for employing binders, matrices or other additives to thecoating. Thus, resins, charcoal matrices, etc. are eliminated, with theresult that the coating may be applied more rapidly than prior coatingmaterials and at much less cost.

Upon the completion of the coating step, the crushed glass particles areintroduced through the conduit 13 to the fluidizing bed 15. The rate offlow of the incoming particles is such that there is continuouslymaintained within the bed channels 30 and 31 (FIGS. 3 and 3a) a volumeof particles that is about one-half the volume of the channels. Theparticles are fluidized in the channels 30 and 31 by the inert gas fromthe conduits 53, 54, 55, 56 and 57, and the particles are heated to anelevated temperature sufficiently high and for a time sufficient tosoften the particles and allow surface tension to shape them intospherical form while in a fluidized condition.

The heating of the particles is carefully controlled as they movethrough the successive zones 36, 37, 38 and 45 of the fluidizing bed 15by regulating the temperature of the inert atmosphere within the zones.This is accomplished by controlling the external heaters 67, 69 and 70and the internal heaters 39, 40 and 41 (FIGS. 3 and 3a). For thespheoridization of soda lime silicate glass, for example, thetemperature of the particles moving through the zone 36 is raised toabout 400° C. At this stage in the process the particles are not yetsoft, and they retain their uniform carbonaceous coating. In the zone 37the particle temperature is again increased, and in zones 38 and 45 thetemperature is further increased to approximately 850° C. or 900° C. Theresidence time in the two zones 38 and 45, illustratively 15 minutes, issufficient to permit each particle to become soft and enable the surfacetension of the particle to shape it into spherical form while beingmaintained in a fluidized condition. The atmosphere within the zones 38and 45 is sufficiently inert to avoid any burning or oxidation of thecoating on the particle. The incoming inert gas from the conduit 55 ismaintained at a temperature of about 600° C. by the heater 69, and theheater 70, together with the heaters 39, 40 and 41 (FIGS. 3 and 3a)provide a further increase in the temperature of the atmosphere withinthe zones 38 and 45 to bring the particles to their spheoridizationtemperature.

The fluidized particles within the bed 15 are held at theirspheoridization temperature as the particles move through the zone 38 tothe outfeed zone 45. As best seen in FIG. 2, the level of the particlesin the zone 45 has dropped substantially as a result of the weir plate46, and the particles proceed over the ledge 48 on the weir plate 47 andinto the vertical discharge conduit 49.

From the discharge conduit 49 the now spherical particles enter thesealer and intermediate cooler bed 105. The particles are subjected to asharp drop in temperature within the bed 105, and they are maintained atthe reduced temperature, illustratively 600° C., in a fluidizedcondition for a period of time sufficient to cause the setting of thespheres. In addition to cooling the particles, the bed 105 provides aseal between the inert atmosphere within the bed 15 and the oxidizingatmosphere within the bed 110.

Upon the exposure of the solidified spherical particles to the oxidizingatmosphere in the bed 110, the carbonaceous coating on the particlesrapidly burns off and is discharged through the vent 116. Because ofthis extremely thin coating each individual particle of coating materialis removed from the surface of the spherical particle with the resultthat the individual spheres are optically clear and require no furthercleaning, washing or other treatment. The oxidizing atmosphere withinthe bed 110 is at a temperature in excess of the burning or oxidationtemperature of the coating material but below the softening temperatureof the spherical particles to avoid sticking or deformation of thespheres as they contact one another after the coating has been removed.The atmosphere within the bed 110 is maintained at this temperature bythe heated air entering the bed through the heater 126 and the conduit115 and by the heat generated by the burning of the coating material.

The optically clear glass spheres proceed through the conduit 128 to thecooler bed 130. The particles are maintained in a fluidized conditionwithin the bed 130 as their temperature is further reduced to about 90°C. The resulting product is then discharged into a collector 148.

During the manufacturing process both the particles and the coatingmaterial are in a dry condition without the presence of water or otherliquids. The presence of water in the tumbler 10, for example, exhibitsa tendency to cause the particles to stick together and alsonecessitates the use of a much heavier coating on each particle. At thetemperatures encountered within the fluidizing bed 15 the water maycause the formation of oxygen with the result that some of the coatingmaterial may burn off prematurely. As the particles touch one another intheir fluidized condition, to avoid sticking or misshapen particles itis important that the coating remain on each particle until such time asthe particles have solidified in the form of glass spheres. The coatingis then removed from the spheres while they are still at an elevatedtemperature.

Referring now to FIGS. 8 and 9, there is shown a fluidizing bed 150 forreceiving a multiplicity of crushed particles to be spheoridized inaccordance with another illustrative embodiment of the invention. Thebed 150 is provided with a casing 152 of refractory heat-insulatingmaterial and a cylindrical shell 153 within the casing. A perforatedbottom plate 154 is located within the shell 153 in spaced relationshipwith the bottom wall of the casing 152. Protruding through the bottomwall is an inlet conduit 155 which is connected to the nitrogen conduit60 (FIG. 1) or other suitable source of heated inert gas. The conduit155 admits gas into the space between the bottom wall and the plate 154and then through the perforations in the plate to maintain the crushedparticles within the shell 153 in a fluidized condition in an inertatmosphere. The shell includes a return conduit 156 for continuouslywithdrawing inert gas from the shell and recycling the gas through thesystem in the manner described above.

A particle inlet conduit 158 extends through the cylindrical side wallof the shell 153, and a burner 160 is externally disposed adjacent theshell. The exhaust from the burner communicates with the interior of theshell 153 through a conduit 161 which is tangentially connected to thelower portion of the shell. A fuel rich flame is maintained in theburner 160 to create soot in the form of carbon black in the burnerexhaust.

As the crushed glass particles are admitted through the conduit 158 intothe shell 153, they become fluidized by the incoming inert gas from theconduit 155. The incoming carbon black from the burner exhaust conduit161 follows a whirling or vortical path as it enters the shell 153 toapply a thin but complete coating to each individual glass particlewithin the shell. The burner flame is adjusted to introduce the carbonblack in the proportions discussed above.

The coated particles within the fluidizing bed 150 are heated to anelevated temperature sufficiently high to allow surface tension to shapethe particles into spherical form while the particles are in a fluidizedcondition within the bed. The particles are then cooled while in afluidized condition for a period of time sufficient to cause the settingof the spheres, and the coating is removed by means of theabove-described oxidization process.

Although the invention has been illustrated and described as havingparticular utility in the manufacture of glass spheres through the useof one or more fluidizing beds, certain features of the invention alsomay be employed in other types of sphere forming systems. For example,the novel coating and coating removal techniques described herein resultin a more efficient process and a substantially improved product whenusing vertical draft tube systems, rotary kilns, so-called frying pantechniques and the manufacture of the spheres by means of a dropping orprilling tower. Because the coating prevents the deformation or stickingof the particles in these various systems and is readily removablewithout the need for washing the spheres, the resulting product exhibitsextremely good uniformity and a much higher percentage of true spheres.Various other sphere producing or treating systems with which theinvention may be employed will become apparent to those skilled in theart upon a perusal of the foregoing specification.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A process for making spheres from a multiplicityof minute particles, the process comprising, in combination:introducinga multiplicity of the particles into a fluidizing bed; directing afluidizing material into the bed to suspend the particles therein andthereby fluidize the same, the fluidized particles moving substantiallyrandomly within said bed but without appreciable vertical movement ofthe mass of particles; heating the particles to an elevated temperaturesufficiently high to allow surface tension to shape the particles intospherical form while in a fluidized condition in said bed; andthereafter cooling the spherical particles while maintaiing theparticles in a fluidized condition for a period of time sufficient tocause the setting of the spheres.
 2. A process for making spheres from amultiplicity of minute particles, the process comprising, incombination:introducing a multiplicity of the particles into afluidizing bed; directing an inert fluidizing gas into the bed tosuspend the particles in an inert atmosphere and thereby fluidize thesame; applying heat to the fluidizing gas to raise the temperature ofthe particles sufficiently high to allow surface tension to shape theparticles into spherical form while in a fluidized condition in saidbed; and thereafter cooling the spherical particles while maintainingthe particles in a fluidized condition for a period of time sufficientto cause the setting of the spheres.
 3. A process as defined in claim 2,in which the fluidizing gas is continuously withdrawn from the bed andis recycled thereto.
 4. A process for making spheres from a multiplicityof minute particles, the process comprising, in combination:introducinga multiplicity of the particles into a first fluidizing bed; directing afluidizing material into the first bed to suspend the particles thereinand thereby fluidize the same, the fluidized particles movingsubstantially randomly within said bed but without appreciable verticalmovement of the mass of particles; heating the particles to an elevatedtemperature sufficiently high to allow surface tension to shape theparticles into spherical form while in a fluidized condition in saidfirst bed; and thereafter transferring said particles to a secondfluidizing bed, the second bed maintaining the particles in a fluidizedcondition for a period of time sufficient to cause the setting of thespheres.
 5. A process for making spheres from a multiplicity of minuteparticles, the process comprising, in combination:applying a protectivecoating to the particles; introducing the particles into a fluidizingbed; directing a fluidizing gas into the bed to suspend the particlestherein and thereby fluidize the same; heating the particles to anelevated temperature sufficiently high to allow surface tension to shapethe particles into spherical form while in a fluidized condition in saidbed; cooling the spherical particles while maintaining the particles ina fluidized condition for a period of time sufficient to cause thesetting of the spheres; and removing said coating from the sphericalparti- cles.
 6. A process for making spheres from a multiplicity ofminute particles, the process comprising, in combin- ation:coating theparticles with an oxidizable protective material; introducing the thuscoated particles into a sphere forming enclosure; heating the particlesto an elevated temperature sufficiently high to shape the particles intospherical form; thereafter cooling the particles for a period of timesufficient to cause the setting thereof to form spheres; and exposingthe spheres to an oxidizing atmosphere to oxidize and remove saidcoating.
 7. A process for making spheres from a multiplicity of minuteparticles, the process comprising, in combina- tion:coating theparticles with an oxidizable adherent protective material, the amount ofcoating material applied to the particles lying within the range of fromabout 0.1% to about 0.5% by weight; introducing the thus coatedparticles into a sphere forming enclosure having an inert atmosphere;heating the particles to an elevated temperature sufficiently high toallow surface tension to shape the particles into spherical form;thereafter cooling said particles for a period of time sufficient tocause the setting thereof to form spheres; and completely removing saidcoating from the spherical particles by oxidizing said coating.
 8. Aprocess for making glass spheres from minute glass particles, theprocess comprising, in combination:coating the glass particles with anoxidizable adherent protective material; introducing the thus coatedparticles into a sphere forming enclosure having an inert atmosphere;heating the particles to an elevated temperature sufficiently high toshape the particles into spherical form while in said enclosure;thereafter cooling said particles for a time sufficient to cause thesetting of the spheres; and exposing the spheres to an oxidizingatmosphere to oxidize and remove said coating.
 9. A process as definedin claim 7, in which the oxidizable adherent protective materialcomprises carbon black.
 10. A process as defined in claim 8, wherein theoxidizable adherent protective material is applied to the particles bytumbling a mixture of the material and the particles.
 11. A process asdefined in claim 8 which further comprises, in combination:providing afuel rich flame; and mixing the products of combustion from said flamewith said particles to provide said coating of oxidizable adherentprotective material thereon.
 12. A process for making spheres from amultiplicity of minute particles, the process comprising, incombination:coating the particles with an oxidizable protectivematerial; introducing the thus coated particles into a first fluidizingbed; directing an inert gas into the first fluidizing bed to suspend theparticles in an inert atmosphere and thereby fluidize the same; heatingthe particles to an elevated temperature sufficiently high to allowsurface tension to shape the particles into spherical form while in afluidized condition in said first bed; transferring said particles to asecond fluidizing bed, the second bed maintaining to continue tomaintain the particles in a fluidized condition for a period of timesufficient to cause the setting of the spheres; and removing saidcoating from the spherical parti- cles.
 13. A process for making glassspheres from minute glass particles, the process comprising, in combin-ation:coating the glass particles with an oxidizable adherent protectivematerial; introducing the thus coated particles into a first fluidizingbed having an inert atmosphere; heating the particles to an elevatedtemperature sufficiently high to allow surface tension to shape theparticles into spherical form while in a fluidized condition in saidfirst bed; cooling said particles while maintaining the particles in afluidized condition for a time sufficient to cause the setting of thespheres; and transferring the particles to a second fluidizing bedhaving an oxidizing atmosphere, the particles being maintained in afluidized condition in said second bed for a period of time sufficientto oxidize and completely remove the protective coating thereon.
 14. Aprocess as defined in claim 13, in which the oxidizable adherentprotective material comprises carbon black.
 15. A process for makingglass spheres from minute glass particles, the process comprising, incombin- ation:coating the glass particles with an oxidizable adherentprotective material; introducing the thus coated particles into a firstfluidizing bed having an inert atmosphere; heating the particles to anelevated temperature sufficiently high to allow surface tension to shapethe particles into spherical form while in a fluidized condition in saidfirst bed; cooling said particles while maintaining the particles in afluidized condition for a time sufficient to cause the setting of thespheres; transferring the fluidized particles to a second fluidizing bedhaving an oxidizing atmosphere, the particles being maintained in afluidized condition in said second bed for a period of time sufficientto oxidize and remove the protective coating thereon; and thereaftertransferring said particles to a third fluidizing bed to further coolthe particles.